diff --git a/lib/manager/npm/post-update/index.js b/lib/manager/npm/post-update/index.js
index d2d409c25a638d93718ce64a233fd3265c343cfc..fc205791f0766391a2e599e4cb67e4b28943ac34 100644
--- a/lib/manager/npm/post-update/index.js
+++ b/lib/manager/npm/post-update/index.js
@@ -170,10 +170,12 @@ async function writeExistingFiles(config, packageFiles) {
       }
     }
     const npmrc = packageFile.npmrc || config.npmrc;
-    if (npmrc.includes('=${' && !config.global.exposeEnv)) {
-      logger.info('Skipping .npmrc as it contains variables');
-    } else {
-      await fs.outputFile(upath.join(basedir, '.npmrc'), npmrc);
+    if (npmrc) {
+      if (npmrc.includes('=${' && !config.global.exposeEnv)) {
+        logger.info('Skipping .npmrc as it contains variables');
+      } else {
+        await fs.outputFile(upath.join(basedir, '.npmrc'), npmrc);
+      }
     }
     if (packageFile.yarnrc) {
       logger.debug(`Writing .yarnrc to ${basedir}`);